Black Sea port partially passed on heading north (6)
I believe the answer is:
odessa
'black sea port' is the definition.
(Ukrainian port on the Black Sea)
'partially passed on heading north' is the wordplay.
'partially' indicates a hidden word.
'heading north' says the letters should be written backwards.
'assedo' can be found hidden inside 'passed on'.
'assedo' written backwards gives 'ODESSA'.
(Other definitions for odessa that I've seen before include "Battleship Potemkin mutiny port" , "Black Sea location" , "Do seas come into Ukrainian city?" , "Port city on the Black Sea" , "Port city of Ukraine" .)